Thermally-assisted photosensitized emission in a trivalent terbium complex

Luminescent lanthanide complexes containing effective photosensitizers are promising materials for use in displays and sensors. The photosensitizer design strategy has been studied for developing the lanthanide-based luminophores. Herein, we demonstrate a photosensitizer design using dinuclear luminescent lanthanide complex, which exhibits thermally-assisted photosensitized emission. The lanthanide complex comprised Tb(III) ions, six tetramethylheptanedionates, and phosphine oxide bridge containing a phenanthrene frameworks. The phenanthrene ligand and Tb(III) ions are the energy donor (photosensitizer) and acceptor (emission center) parts, respectively. The energy-donating level of the ligand (lowest excited triplet (T1) level = 19,850 cm−1) is lower than the emitting level of the Tb(III) ion (5D4 level = 20,500 cm−1). The long-lived T1 state of the energy-donating ligands promoted an efficient thermally-assisted photosensitized emission of the Tb(III) acceptor (5D4 level), resulting in a pure-green colored emission with a high photosensitized emission quantum yield (73%).

The organic ligands undergo intersystem crossing (ISC) from the lowest singlet excited state (S 1 ) to the lowest triplet excited state (T 1 ) after excitation, thereby transferring their electronic energy to the Ln(III) ion. Latva et al. conducted a detailed investigation of the relationship between the photosensitized emission efficiency and T 1 level using green luminescent Tb(III) and amino-carboxylatetyped ligands 14 . They suggested that the energy of the T 1 level should be enough higher than that of the emitting level of Tb(III) ions ( 5 D 4 : 20,500 cm −1 , Supplementary Note 1 and Fig. S1) for strong Ln(III) emission ( Fig. 1a; required energy gap between donor and acceptor in case of Tb(III) complexes >1850 cm −1 (Latva's empirical rule)). This photosensitized energy transfer system, requiring a high T 1 level, causes a strong restriction of the organic ligand designs in lanthanide complexes [15][16][17][18] .
Herein, we focused on the long-lived excited organic donor system to break this photosensitizer design rule of luminescent Ln(III) complexes. Theoretical calculations have suggested that the T 1 -Ln(III) energy transfer rate is much higher than the inverse of the lifetime of the excited states of Ln(III) ions 19 . The long T 1 lifetime should allow the efficient use of Ln(III) emitting photons, even in the case of a low T 1 level, when an excited equilibration between T 1 and Ln(III) emitting states is formed [20][21][22][23][24][25][26] . In this study,  we demonstrated the photosensitized emission of the Tb(III)  complex with a T 1 level of an organic ligand lower than the Tb(III)  emitting level for the first time, using the long-lived excited organic  ligands (Fig. 1b and Supplementary Note 2).
To demonstrate our conceptual strategy, we designed the sevencoordinated Tb(III) complexes with a 2,2,6,6-tetramethyl-3,5heptanedionate (tmh) and bidentate phosphine oxide-containing phenanthrene framework (dpph, Fig. 1c). A density functional theory calculation indicated that the T 1 level of dpph is lower than that of a Tb(III) ion. The phosphine oxide-containing polyaromatic hydrocarbon framework also provides long-lived localized T 1 states in lanthanide complexes, which function as effective energy donors 20,27,28 . Two-sided tmh ligands encapsulate the dpph ligand, thereby extending dpph's T 1 lifetime 29 . The Lu(III) complex with a closed 4f-electronic configuration was prepared to estimate dpph's energy level and excited lifetime in an Ln(III) complex (Fig. 1d) 30 . The photosensitization mechanism presents new frontiers in the fields of molecular lanthanide photophysics and photofunctional material science.

Results and discussion
Coordination structure. The Tb(III)-Tb(III) and Lu(III)-Lu(III) dinuclear complexes were prepared by the complexation of [Tb 2 (tmh) 6 ] and [Lu(tmh) 3 ] with dpph in methanol, respectively (where [Tb 2 (tmh) 6 (dpph)]: Tb-dpph, and [Lu 2 (tmh) 6 (dpph)]: Lu-dpph). Single crystals of the dinuclear Tb(III) complex were obtained by recrystallization from the methanol solution. The crystal structure of Tb-dpph, shown in Fig. 2, was found to be triclinic, with the space group being P-1 (for the crystallographic data, see Table S1, ESI †). The coordination site in the Tb(III) complex comprised three tmh ligands and one phosphine oxide ligand. The single-crystal structure of the as-obtained Lu-dpph is almost the same as that of Tb-dpph (Fig. S2).
Photophysical properties of ligand-excited states. The emission spectrum of Lu-dpph, in degassed condition, is shown in Fig. 3a (solid line). The Lu-dpph shows a broad band at around 550 nm, which originated from π-π* transition of dpph ligand moiety (Supplementary Note 3 and Fig. S3-S5). The emission spectrum was deconvoluted into three vibronic bands using the software (OriginPro 2021b), the spectrum in wavenumber scale, and by fitting the peak profile using Gaussian functions (Fig. 3a, broken line). The deconvolution results in the three vibronic bands were designated as 0-0 (19,850 cm −1 ), 0-1 (18,670 cm −1 ), and 0-2 (17,390 cm −1 ). Thus, the T 1 level of the dpph ligand in Lu-dpph  was determined to be 19,850 cm −1 using band-deconvolution analysis. The emission photograph of Lu-dpph is shown in Fig. 3b, where it shows a green persistent luminescence.
Photophysical properties of a trivalent terbium complex. The emission and excitation spectra of Tb-dpph in degassed conditions are shown in Fig. 4a. Sharp emission bands at 490, 548, 583, 616, 651, and 679 nm were observed for Tb-dpph, which are assigned to the 5 D 4 → 7 F 6 , 5 D 4 → 7 F 5 , 5 D 4 → 7 F 4 , 5 D 4 → 7 F 3 , 5 D 4 → 7 F 2 , and 5 D 4 → 7 F 1,0 transitions of Tb(III), respectively. The observed excitation spectral bands at 344 and 362 nm are consistent with the absorption bands of the dpph ligand (Fig. S9), indicating energy transfer from the π-conjugated dpph ligand to Tb(III). The emission quantum yield and emission lifetime of Tb-dpph excited by the dpph ligand are estimated to be 73% and 0.83 ms, respectively. Thus, we successfully demonstrated a strong photosensitized emission using the energy-donating ligand with a lower T 1 level than the emitting level of Tb(III).
Mechanistic study. To understand this characteristic energy migration system, we evaluated the photophysical properties of the Tb-dpph excited by dpph ligand under the presence of oxygen. An excited state equilibrium between Tb(III) and ligand T 1 was revealed through the emission lifetime measurements based on the oxygen concentrations 20,[22][23][24][25][26] (Fig. S10, Ar: 0.83 ms, Air: 0.57 ms). The photosensitized emission quantum yield was also dependent on the oxygen concentrations (Ar: 73%, Air: 57%). The energy diagram for Tb-dpph is shown in Fig. 4b. From the fluorescence measurements (Fig. S11), the S 1 level of the dpph ligand (27,100 cm −1 ) is lower than that of the tmh ligand (30,400 cm −1 ). These results demonstrate that the effective photosensitized energy transfer occurs via the T 1 state of the dpph moiety in the Tb(III) complex. The T 1 level of the dpph ligand (19,850 cm −1 ) is much lower than that of the tmh ligand (24,400 cm −1 ) 37 , hence indicating that the energy transfer pathway from the dpph to the tmh ligand is negligible. To further understand the excited state dynamics, we evaluated the temperature dependence of the emission intensity and 4f-4f emission lifetimes (Supplementary Notes 5, 6 and Figs. S12-18). The photosensitized emission intensity increased with the temperature from 100 to 400 K, suggesting the existence of a thermally-enhanced photosensitization pathway such as intersystem crossing 38 and/or energy transfer from T 1 . The temperature-dependent emission measurement by direct 4f-4f excitation revealed the existence of a thermally-enhanced emission via the T 1 state in the excited-state equilibrium. The results suggest the existence of an endothermic energy transfer pathway corresponding to the 7 F 6 → 5 D 4 transitions (Supplementary Note 7, Fig. S19, and Table S3). However, time-resolved emission spectroscopy showed a temperature-insensitive emission lifetime (100-350 K) at the excited-state equilibrium with the long-lived excited state of the dpph ligand (Supplementary Note 6 and  Figs. S16-18). The results suggest unusually efficient exothermic energy transfer pathways corresponding to the 7 F 5 → 5 D 4 transitions from the T 1 states (+ΔE = 1400 cm −1 ) besides the endothermic energy transfer pathways corresponding to the 7 F 6 → 5 D 4 transitions from the T 1 states (−ΔE = 650 cm −1 ). Theoretical studies suggest significantly populated 7 F 5 owing to the long decay lifetime of 7 F 5 → 7 F 6 in a relatively large energy gap between them (ca. 2050 cm -1 ) 39,40 , allowing energy transfer from 7 F 5 level 41 . Theoretical studies also indicate a larger energy-transfer matrix element for the 7 F 5 → 5 D 4 transition than that for the 7 F 6 → 5 D 4 transition 42 . The energy transfer from the 7 F 5 state is one of the models for explaining the present temperature-insensitive lifetime behavior (the detailed discussion in Supplementary Note 6). Considering the temperature-dependent photophysical measurements and theoretical aspects, the characteristic thermally-assisted photosensitized emission occurs via the dpph T 1 state. Although determining the exact photosensitization pathway is difficult, this is, to the best of our knowledge, the first example of efficient photosensitized emission via the T 1 state in a lanthanide complex with an organic ligand T 1 level lower than the emitting level of the Ln(III) ion (Supplementary Note 8 and Figs. S20, S21).

Conclusions
In this study, an effective photosensitized emission in a luminescent lanthanide complex with a T 1 level of an organic ligand lower than the emitting level of an Ln(III) ion was demonstrated. The thermally-assisted photosensitized emission was based on the excited-state equilibrium between a luminescent Ln(III) ion and an organic ligand with a persistent excited state. The photosensitizer model with a low T 1 level is advantageous for the construction of low-energy-driven photosensitization (Supplementary Note 9 and Fig. S22). The present study not only breaks the historical photosensitizer design rule based on Latva's rule, but also presents a novel photosensitizer model for photofunctional materials beyond lanthanide photochemistry.
Preparation of [2,7-bis(diphenylphosphoryl)phenanthrene (dpph)]. A solution of n-butyllithium (6.2 mL, 9.9 mmol) was added dropwise to a solution of 2,7-dibromophenanthrene (1.67 g, 4.97 mmol) in dry tetrahydrofuran (45 mL) at -76°C under Ar atmosphere. After 2 h, chlorodiphenylphosphine (1.8 mL, 9.8 mmol) was added to the solution at -76°C under Ar atmosphere, and then stirred for 20 h at room temperature. The reaction mixture was added to dichloromethane, washed with water, and then dried over anhydrous sodium sulfate. The obtained solution was evaporated and chloroform (30 ml) was added to the product. A 30% hydrogen peroxide aqueous solution (4 mL) was added to the solution, and the reaction mixture was stirred for 2 h. The product was extracted using dichloromethane, and the extract was washed with water and then dried over anhydrous sodium sulfate. The compounds were purified by silica gel column chromatography (ethyl acetate: methanol = 23: 2) (Yield: 63.8%, 1.83 g, 3.16 mmol). 1  Single-crystal X-ray structure determination. X-ray crystal structures for [Tb 2 (tmh) 6 (dpph)] and [Lu 2 (tmh) 6 (dpph)] are shown in Fig. 2 and Fig. S2, respectively. The crystallographic data are shown in Table S1. Single crystal X-ray diffraction data were obtained using Rigaku XtaLAB Synergy-DW equipped with a HyPix-6000HE detector (MoK α radiation, λ = 0.71073 Å). Non-hydrogen atoms were refined anisotropically using the SHELX system. Hydrogen atoms were refined using the riding model. All calculations were performed using the crystal structure crystallographic and Olex 2 software package. The CIF data were confirmed by the check CIF/PLATON service.

Data availability
The single-crystal data generated in this study have been deposited in The Cambridge Crystallographic Data Center under accession code CCDC-2128731 (for [Tb 2 (tmh) 6 (dpph)], Supplementary Data 1) and CCDC-2128735 (for [Lu 2 (tmh) 6 (dpph)], Supplementary Data 2). These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif. All of the other data supporting the findings of this study are available from the corresponding author upon reasonable request.